The present invention relates to the art of analyzing and monitoring rotating machines. It finds particular application in conjunction with the analysis of vibration signals obtained from the rotating machine to produce a frequency spectrum from which a machine running speed and rolling element bearing condition is determined. It is to be appreciated, however, that the invention will also find application in other fields where the obtaining of a rotating object's speed is desired.
As is well known in the machine analysis and monitoring arts, defects in rotating machines can be analyzed through the use of a vibration frequency spectrum. The presence of vibration peaks at certain frequencies, which are known as defect frequencies, are indicative of a specific machine problem. As is shown in REFERENCE CHART 1 below, a common type reference used in the industry, defect frequencies are calculated as multiples of the running speed of the machine. For example, a machine rotor that is out of balance will generate a vibration at 1.0 times running speed of the machine, and a bent shaft will generate a vibration at 1.0 or 2.0 times running speed. Similarly, through calculation such as that of Equation 1 below, a rolling element bearing having a defective inner race can be found which may for example, generate a vibration at 5.63 times the running speed of the machine. Similar calculations are equally obtainable for the outer race defects as shown by Equation 2.
__________________________________________________________________________ REFERENCE CHART 1 - VIBRATION SOURCES PROBABLE FREQUENCY relative STROBE CAUSE to machine RPM "PICTURE" AMPLITUDE NOTES __________________________________________________________________________ Unbalance RPM .times. 1 One - Steady Radial - Steady Most Common cause of vibration Bent Shaft RPM .times. 1 (or .times. 2) 1, 2, or 3 Axial - high Strobe picture depends on machine usually unsteady Sleeve Bearings RPM .times. 1 One - Steady Shaft = Compare shaft to Bearing Rdgs. bearing readings Faulty Belts RPM .times. 1 to RPM .times. 5 See "Notes" Radial - Unsteady Freeze belts with (Belt RPM .times. 2) Column strobe and observe Oil Whip Less then Unstable Radial - Unsteady Frequency is near machine RPM sometimes severe 1/2 RPM (commonly 42% to 48% .times. RPM) Gears High (related to -- Radial - low Use velocity or number of gear teeth) acceleration mode Looseness RPM .times. 2 2 Proportional Frequency coupled to looseness with misalignment Foundation Failure Unsteady Unstable Erratic Shows up when balancing Resonance Specific "criticals" 1 High Increased levels at critical speeds Beat Frequency Periodically varying -- Pulsating Caused by close RPM machines Misalignment Parallel RPM .times. 1, .times. 2 1, 2, or 3 Radial Angular RPM .times. 1, .times. 2 1, 2, or 3 Axial - high Axial amplitude = .7 or higher of vertical or horizontal __________________________________________________________________________ Equation 1: ##STR1## Equation 2: ##STR2## Where: IR = Inner Race Defect OR = Outer Race Defect N = number of rotating elements d = diameter of rotating elements Pd = pitch diameter of rotating elements f.sub.o = fundamental frequency of machine (Running Speed)
Therefore as can be seen from REFERENCE CHART 1 and Equations 1 and 2, machine analysis relies heavily on having an accurate knowledge of the machine's speed. A minor error in determining running speed would result in the inability to determine whether a specific examined frequency was truly a multiple of running speed, or a random frequency. In the prior art, obtaining the machine speed with the accuracy required was normally accomplished through the use of a speed sensor, see for example U.S. Pat. No. 4,426,641, METHOD AND APPARATUS FOR MONITORING THE SHAFT VIBRATION OF A ROTARY MACHINE, Kurihara, et al. Various means of speed sensing were used by the prior art including the rotary pulse gear 7 disclosed in FIG. 1c of Kurihara, et al.
Thus, due to the necessity of knowing a machine's speed accurately, a separate speed sensor in addition to a transducer, such as a vibration type, was required. The cost of separate speed and vibration sensors on a large machine such as a turbine-generator is no major problem. However, incorporating both a speed sensor and a vibration sensor on typical pumps, motors and fans does have a major impact. For example, in a typical refinery, chemical plant or other facility, literally thousands of machines may require monitoring. Installing a speed sensor on each of these machines significantly increases the installation costs and the maintenance costs of the monitoring equipment. Due to the increased costs for maintaining speed sensors on these machines, many machines which should be monitored are not.
Additionally, in the area of rolling element bearing defect analysis, two major problems exist. First, the amplitude of the vibration produced by the bearing defect is extremely small when compared to other vibrations produced in a rotating machine. Second, in order to calculate the bearing defect frequencies, the operator must know the specific geometry of the bearings, i.e. pitch diameter of the bearing, ball diameter in the bearing, number of balls in the bearing, and contact angle of the bearing, in addition to the running speed of the machine. Once the bearing has been installed within a machine, it is often impossible to ascertain the actual geometry of the bearing. Once installed in an operating machine, the parameters cannot be measured in a factory environment. Moreover, the machine records are often not detailed enough to reveal which specific bearing may be installed in any one of the many machines that are in service. As a complication, the manufacturer of standard machinery may buy the bearings from different manufacturers. Although the bearing fits the same machine, each commonly has different internal physical geometries.
Therefore, one of the drawbacks of prior art machine analysis and monitoring is a requirement of including hardware to maintain a separate speed sensing device in order to obtain an accurate determination of the running speed of the rotating machine.
Another drawback of the prior art systems which attempt to employ vibration signals to estimate rolling element bearing defects, is a requirement of knowing the specific geometry of the bearing which is being analyzed.
The present invention provides a new and improved method and apparatus for analyzing rotating machines which overcomes the above-referenced drawbacks and others. This satisfies a long-felt need in the industry to determine the running speed of a machine without the requirement of a separate speed transducer or sensor. Detailed frequency analysis is allowed on rotating machines with only an affixed vibration transducer.